When using a laser to cut workpieces, in particular when laser cutting thin substrates out of, for example, glass, sapphire or similar materials, damage is known to occur not only in the immediate vicinity of the material ablation on the upper side of the workpiece facing the laser beam, but also so-called secondary processing zones normally at some distance can form inside the workpiece material or on the underside of the workpiece, in which the workpiece material is damaged by the laser process (e.g., modified locally such as melted). The cause of the formation of these secondary processing zones is assumed to be, amongst other things, undesired optical effects, such as reflections of the laser beam when laser cutting inside the optically transparent workpiece material being cut. The extent to which the secondary processing zones are formed varies also depending on the material and/or on the dimensions of the workpiece, as well as on the process parameters used in the laser cutting.
The secondary processing zones described above form not only an optical defect in the processed workpiece, but they also affect its stability disadvantageously. For example, the mechanical resilience of the processed workpiece is lowered by the secondary processing zones and consequently in particular the maximum bearable bending stress (bending stress to fracture) of the workpiece is reduced. The further the damage is from the actual cutting region, the less pronounced it is as a rule.
In order to reduce the effect of the material processing on a region adjacent to the actual processing region, EP 1 945 401 B1 provides a method of material separation which uses a laser, where the material region being processed is irradiated with a series of laser pulses. In doing so, the region irradiated by at least two consecutive laser pulses can overlap spatially in the region by 10% to 99%. When cutting through the workpiece along a kerf, at the start of the separation process, initially high laser energy is applied, after which the laser energy is reduced.
EP 2 258 512 B1 provides a method and a device for separating a substrate, where multiple pulses are aimed at the substrate which are able to melt out the substrate locally. By so doing, a structurally weaker region is formed in the interior of the substrate wherein the individual laser pulses can overlap each other spatially. Then, the substrate can be separated (broken) or cut by mechanical action in the weakened region.
U.S. Pat. No. 6,552,301 B2 discloses a method in which a series of laser pulses are fired at a workpiece material. Here, the laser pulses have a very short pulse duration and a flow density which exceeds a threshold to modify the workpiece material.
WO 2012/006736 A2 provides a method for preparing a splitting of a substrate, where the substrate is irradiated with one or multiple laser pulses, so that initially a first filament appears in the substrate. By having a relative movement between substrate and laser beam, a second additional filament is produced at another place in the substrate, so that the filaments form a pattern along which the substrate is split.